Electrode for electrochlorination

ABSTRACT

The invention relates to an electrode for electrochemical generation of hypochlorite. The electrode comprises a valve metal substrate coated with a catalytic system consisting of two super-imposed layers of distinct composition and having a different activity towards hypochlorite anodic generation from chloride solutions. The electrode has a high duration in cathodic operation conditions, imparting self-cleaning characteristics thereto when used in combination with an equivalent one with periodic polarity reversal. Moreover, the deactivation of the electrode at the end of its life cycle occurs in two subsequent steps, allowing to schedule the substitution thereof with a significant notice period.

FIELD OF THE INVENTION

The invention relates to an electrode for electrochemical generation ofhypochlorite.

BACKGROUND OF THE INVENTION

The electrolytic production of hypochlorite from diluted brines ofalkali metal chlorides, e.g. of sodium hypochlorite by electrolysis ofaqueous solution of sodium chloride or of sea-water, is one of the mostcommon processes in the domain of industrial electrochemistry. Theproduction of hypochlorite is always accompanied by the generation ofvarious by-products deriving from the oxidation of chlorides (generallygrouped under the name of “active chlorine”) and in some cases ofoxygenated species such as peroxides, most of which have a very limitedlifetime; for the sake of brevity, in the present text the whole of suchproducts in aqueous solution, mostly consisting of alkali metalhypochlorite and hypochlorous acid in a ratio mainly depending on pH, isindicated as hypochlorite. Depending on the production volumes andconcentrations, product hypochlorite may be used in several ways, forinstance in paper and cloth bleaching, in the disinfection of drinkingor pool water or for domestic uses. Potassium hypochlorite is alsoemployed in preventive or therapeutic treatment of agriculturalcultivations. Hypochlorite is generally produced in undividedelectrolytic cells with electrodes of various shapes and geometry, forexample with interleaved planar electrodes. In an electrolytic cell,hypochlorite production takes place by anodic oxidation of chloride,while hydrogen is evolved at the cathode; when the chloride solution tobe electrolysed contains sensible amounts of calcium or magnesium ions,such as the case of civil water chlorination, the natural alkalinisationof the electrolyte in the proximity of the cathode surface causes thelocal precipitation of limestone, which tends to deactivate the cathodesand prevent their operability after some time. Among the varioussolutions proposed to obviate this problem, one of the most effectiveconsists of submitting the electrodes to cyclic potential reversal,alternating their use as cathodes and as anodes. In this way, thecarbonate deposit which settles on the surface of an electrode undercathodic operation is dissolved during the subsequent operation asanode, in which condition the surrounding environment tends to acidify.Since hydrogen evolution takes place at sufficiently low potential onmany metallic materials, the electrodes of an electrochlorinatordesigned to work under alternate electrodic polarisation are activatedwith a catalyst aimed at maximising the efficiency of the hypochloritegeneration anodic reaction, more critical both in terms of overpotentialand of selectivity due to the concurrent, undesired oxygen evolutionanodic reaction. A catalyst whose efficiency is well known in thisprocess comprises a mixture of oxides of noble metals (typicallyruthenium and optionally iridium or palladium) and of valve metaloxides, preferably titanium oxide. Such catalyst is applied according tovarious methodologies to a substrate typically made of titanium, whichallows obtaining an electrode configuration capable of working ascathode for hydrogen evolution with a good efficiency. The noble metalin the catalytic formulation has the main purpose of catalysing theanodic reaction and is bound to the valve metal in a solid solutionwhich contributes to reduce the consumption thereof; other valve metals,such as tantalum and niobium, might be used to replace titanium,although they are considered a less valid alternative due to theirtendency to decrease the anodic reaction selectivity thereby producing ahigher amount of oxygen with a net loss of hypochlorite.

The functioning of the electrodes in alternate polarisation conditionsallows operating with good efficiency while keeping the electrodesurface sufficiently clean from insoluble deposits; nevertheless,cathodic operation under hydrogen evolution of such type of electrodeconfigurations entails a reduced operative lifetime, because theadhesion of the coating to the substrate tends to be hampered in theseconditions. The deactivation mechanism of this type of electrodes,mainly associated with the consumption of the catalyst layer or thedetachment thereof from the substrate, brings about a sudden failurewith no significant premonitory sign; in order to prevent seriousinconveniences, an estimation of residual lifetime of the electrodes ina cell is usually carried out on a statistical basis and theirreplacement is scheduled before a quick and irreversible failure occurs.Since the deactivation of electrodes working under this kind ofoperative conditions is affected by several factors, the variability israther high, so that keeping a sufficient margin of safety often impliesreplacing electrodes which might have been functioning for a significantresidual time.

It has been thus evidenced the need for providing a new electrodecomposition for operation in alternate polarity conditions inelectrolytic processes with production of hypochlorite, characterised byan equal or higher overall duration with respect to prior artformulations and by a deactivation profile allowing to convenientlyschedule their substitution by better forecasting their residuallifetime.

SUMMARY OF THE INVENTION

Various aspects of the invention are set out in the accompanying claims.

In one embodiment, an electrode for hypochlorite generation comprises asubstrate made of a valve metal, typically titanium optionally alloyed,having a suitable roughness profile, an internal catalytic coating andan external catalytic coating of different composition and higheractivity overlying the internal catalytic coating, the roughness profilebeing characterised by R_(a) comprised between 4 and 8 μm and R_(z)comprised between 20 and 50 μm, the internal catalytic coatingcontaining oxides of iridium, ruthenium and a valve metal selectedbetween tantalum and niobium with an overall specific loading of iridiumplus ruthenium expressed as metals of 2 to 5 g/m², the externalcatalytic coating containing noble metal oxides with an overall specificloading not lower than 7 g/m². In one embodiment, the external catalyticcoating comprises a mixture of oxides of iridium, ruthenium and titaniumwith a molar concentration of ruthenium of 12-18%, a molar concentrationof iridium of 6-10% and a molar concentration of titanium of 72-82%. Aroughness profile as indicated, characterised by rather deep cavitiesrelatively spaced apart, allows overlying two distinct catalyticcoatings so that the innermost coating, strongly anchored within thecavities, starts working only upon completion of the detachment of theoutermost coating. The inventors surprisingly observed that theelectrode as hereinbefore described is characterised by a two-stepdeactivation mechanism, with a first voltage increase with respect tothe normal operative voltage up to sensibly higher values still suitablehowever to continue its operation (for instance a voltage increase of500-800 mV) and a second, quicker voltage increase forcing itsdefinitive shut-down. Without wishing to limit the present invention toany particular theory, it might be supposed that a configurationproviding an internal coating not too efficient towards hypochloritegeneration, such as a combination of oxides of noble metals as iridiumand ruthenium in admixture with an oxide of tantalum and/or niobium, andan external coating of higher performances, such as a mixture of oxidesof iridium, ruthenium and titanium, allows the electrode to operate atexcellent voltage levels until the external coating, convenientlyprovided at a higher specific loading, is present on the electrodesurface; once the external coating is worn off, the internal coatinggets uncovered. The internal coating, which can have a limited specificloading but which is extremely well anchored to the surface due to thespecially selected roughness profile, is capable of operating, althoughat lesser efficiency and higher cell voltage, for a sufficientlyprolonged period of time that permits scheduling the substitution of thewhole cell or of the electrodes preventing the risk of a sudden failure.In one embodiment, the internal catalytic coating contains a mixtures ofoxides of iridium, ruthenium and tantalum with a ruthenium molarconcentration of 42-52%, an iridium molar concentration of 22-28% and atantalum molar concentration of 20-36%. The indicated compositionalranges turned out to be particularly suitable for the formulation ofelectrodes characterised by a deactivation profile which allowsforecasting the need of scheduling a replacement intervention withsufficient anticipation. In one embodiment, the electrode ashereinbefore described comprises a thin protective layer of valve metaloxides, for instance a mixture of oxides of titanium and tantalum,interposed between the substrate and the internal catalytic layer. Thiscan have the advantage of protecting the substrate from passivationphenomena, improving its overall duration without fundamentallyaffecting the characteristic two-step deactivation mechanism.

Under another aspect, a method for manufacturing an electrode ashereinbefore described comprises:

-   -   forming the desired roughness profile by thermally treating the        substrate at a temperature not lower than 550° C., for instance        at 590° C., for a period of at least 4 hours, for instance 5        hours, followed by an acid etching;    -   sequentially applying the internal catalytic layer and then the        external one by thermal decomposition of suitable precursor        solutions.

The thermal treatment of the substrate followed by etching according tothe indicated parameters has likely the effect of segregating theimpurities of the substrate in correspondence of the crystalline grainboundaries; in this way, grain boundaries become a zone of preferentialattack for the subsequent etching. This can have the advantage offavouring the formation of a roughness profile consisting of deep andrelatively spaced apart peaks and valleys, so as to efficaciously anchorthe internal catalytic layer even at reduced specific loadings of noblemetal. The treatment can be carried out in a common oven with forced airventilation; at the end of the thermal treatment, the substrate can beallowed to cool down slowly in the oven and extracted when thetemperature goes below 300° C. In one embodiment, particularly suited totitanium and titanium alloy substrates, the acid etching is carried outwith 25-30% by weight sulphuric acid containing 5 to 10 g/l of dissolvedtitanium, at a temperature ranging from 80 to 90° C. until reaching aweight loss not lower than 180 g/m² of metal. The inventors observedthat these conditions are particularly favourable for the preferentialattack of impurities segregated on the grain boundary during theprevious thermal treatment, facilitating the achievement of the desiredroughness profile. The etching bath can be put into service with adissolved titanium concentration of about 5 g/l and used until thetitanium concentration reaches about 10 g/l by effect of the dissolutionof the substrate itself, then reintegrated with a fresh acid additionuntil the concentration of dissolved titanium is brought back to theoriginal value of about 5 g/l. The titanium in the solution favours thekinetics of dissolution of the valve metal in the etching phase:concentrations below 5 g/l are associated with a dissolution rate whichresults too slow for practical purposes. On the other hand, an excessiveconcentration slows down the attack again. A thermally-treated titaniumsubstrate subjected to an etching step as described reaches a weightloss of 180-220 grams per square metre of surface, a value considered tobe suitable for the subsequent catalytic coating deposition, in a 2 to 3hour time. In one embodiment, the thermally-treated substrate can besubjected to sandblasting before the acid etching. In one embodiment,the substrate subjected to the desired thermal treatment, the optionalsandblasting and the etching is provided with a this protective layerconsisting of valve metal oxides prior to the application of catalyticcoatings. The protective layer can be applied by means of a secondthermal treatment of the valve metal substrate in air, with growth ofthe corresponding oxide, or by an application, for instance of titaniumand/or tantalum oxide, by flame or plasma spraying, or by thermaldecomposition of a suitable precursor solution. The formation of theinternal catalytic layer can be carried out by application andsubsequent thermal decomposition, optionally in multiple coats, of aprecursor solution containing salts or other compounds of iridium,ruthenium and at least one valve metal selected between tantalum andniobium, until reaching an overall loading of 2-5 g/m² of noble metaldefined as sum of iridium and ruthenium expressed as metals. Theformation of the external catalytic layer can be carried out byapplication and subsequent thermal decomposition, optionally in multiplecoats, of a precursor solution containing salts or other compounds ofnoble metals, for instance iridium and ruthenium, and of at least onevalve metal, for instance titanium, until reaching an overall loading ofat least 7 g/m² of noble metal defined as sum of iridium and rutheniumexpressed as metals.

Under another aspect, an electrochemical cell for production ofhypochlorite from a chloride-containing aqueous electrolyte comprisespairs of electrodes as hereinbefore described and a timed control usedto polarise the electrodes alternatively so as to determine theoperation of one electrode of the pair as cathode and of the other asanode and to cyclically reverse their polarity after a predeterminedperiod of time, which in one embodiment is comprised between 30 secondsand 60 minutes. In one embodiment, the aqueous electrolyte is an alkalimetal chloride solution, for instance sodium chloride or potassiumchloride or a mixture of the two, with a chloride ion concentration of 2to 20 g/l.

The following examples are included to demonstrate particularembodiments of the invention, whose practicability has been largelyverified in the claimed range of values. It should be appreciated bythose of skill in the art that the compositions and techniques disclosedin the examples which follow represent compositions and techniquesdiscovered by the inventors to function well in the practice of theinvention; however, those of skill in the art should, in light of thepresent disclosure, appreciate that many changes can be made in thespecific embodiments which are disclosed and still obtain a like orsimilar result without departing from the scope of the invention.

EXAMPLE 1

Two titanium sheets of 10 cm² area and 0.5 mm thickness were washed withhot water and soap, rinsed with deionised water and degreased withacetone. A thermal treatment was subsequently carried out on the sheetsin a forced air ventilation oven at 590° C. for 5 hours. Thethermally-treated samples were allowed to cool in the same oven down toa temperature of 290° C., then extracted, weighed and subjected to anetching treatment on 27% H₂SO₄ containing 5 g/l of titanium at atemperature of 87° C. After two hours of treatment, the samples wereagain washed, dried and weighted, recording a titanium loss of about 200g/m². A roughness profile check, carried out with a Mitutoyo SJ-301profilometer, evidenced an R_(a) value of 5.2-5.3 μm and an R_(z) valueof about 32 μm.

On the two faces of the thus treated sheets an internal catalyticcoating was applied by thermal decomposition of a first precursorsolution containing 47% molar Ru, 24.7% molar Ir and 28.3% molar Ta. Theprecursor solution was obtained starting from a 20% by weight commercialsolution of RuCl₃, a 23% by weight commercial solution of H₂IrCl₆ and asolution of TaCI₅ at a concentration of 50 g/l obtained by dissolutionof solid TaCl₅ in 37% by weight HCl by heating under stirring andsubsequent dilution with water and commercial 2-propanol. The componentswere mixed under stirring, first adding a weighted amount of H₂IrCl₆solution, then the corresponding amount of RuCl₃. After stirring for 30minutes, the solution of TaCl₅ was added and after 30 more minutes themixture was brought to volume with 2-propanol, protracting the stirringfor 30 minutes more. The thus obtained precursor solution was applied tothe titanium sheets, previously dried on air at 50° C., by brushing in 3coats, with a subsequent decomposition cycle in forced air ventilationoven at 510° C. for a time of 10 minutes after each intermediate coatand of 30 minutes after the final coat.

A subsequent weight check evidenced the application of an internalcatalytic coating of 3 g/m² of noble metal, expressed as the sum of Irand Ru.

On the two faces of the sheets an external catalytic coating was appliedby thermal decomposition of a second precursor solution containing 15%molar Ru, 7.9% molar Ir and 77.1% molar Ti. The second precursorsolution was obtained starting from the same reagents used for the firstone, with the addition of commercial TiOCl₂ at 160-180 g/l as areplacement for TaCl₅. Also the preparation was carried out by mixingunder stirring exactly as in the previous case, except that TiOCl₂ andsoon after 18% HCl were added instead of the solution of TaCl₅.

The second precursor solution was applied to the titanium sheets,previously dried on air at 50° C., by brushing in 14 coats, with asubsequent decomposition cycle in forced air ventilation oven at 510° C.for a time of 10 minutes after each intermediate coat and of 30 minutesafter the final coat.

A subsequent weight check evidenced the application of an externalcatalytic coating of 12 g/m² of noble metal, expressed as the sum of Irand Ru.

The thus obtained electrodes were characterised in an acceleratedlife-test under hypochlorite production with periodic polarity reversal.The accelerated test is carried out at a current density of 1 kA/m² inan electrolyte consisting of an aqueous solution containing 4 g/l ofNaCl and 70 g/l of Na₂SO₄, adjusting the temperature at 25±1° C. andreversing the polarity of the two electrodes after every 60 seconds. Insuch operative conditions, much exasperated in comparison with theindustrial application, a constant behaviour was observed with a cellvoltage around 3 V for approximately 300 hours, followed by aprogressive cell voltage increase stabilised, after a total of 400hours, at a new constant value, about 800 mV higher than the previousone. It was nevertheless still possible to operate the cell, albeit at ahigher voltage, after a total 600 hours of test.

EXAMPLE 2

Example 1 was repeated in identical conditions, save for the use of afirst precursor solution containing 47% molar Ru, 24.7 molar Ir and 28.3molar Nb, obtained by replacing the TaCl₅ solution with a 1M solution ofNbCl₅. The obtained electrodes were characterised in the acceleratedlife-test of Example 1, which gave substantially equivalent results thanin the previous example, with a constant cell operation at about 3 Vcell voltage for approximately 215 hours, followed by a progressivevoltage increase stabilised after a total of 320 hours, at a newconstant value, about 600 mV higher than the previous one. In this casethe test was protracted for a total of 400 hours, with some shifting ofthe cell voltage to higher values in the course of the last 40 hours.

COUNTEREXAMPLE

Two titanium sheets of 10 cm² area and 0.5 mm thickness were washed withhot water and soap, rinsed with deionised water and degreased withacetone. A thermal treatment was subsequently carried out on the sheetsin a forced air ventilation oven at 590° C. for 5 hours. Thethermally-treated samples were allowed to cool in the same oven down toa temperature of 290° C., then extracted, weighed and subjected to anetching treatment on 27% H₂SO₄ containing 5 g/l of titanium at atemperature of 87° C. After two hours of treatment, the samples wereagain washed, dried and weighted, observing a titanium loss of about 200g/m². A roughness profile check, carried out with a Mitutoyo SJ-301profilometer, evidenced an R_(a) value of 5.4-5.6 μm and an IR, value ofabout 34 μm.

On the two faces of the thus treated sheets a catalytic coating wasapplied by thermal decomposition of a precursor solution containing 15%molar Ru, 7.9% molar Ir and 77.1% molar Ti, equivalent to the secondprecursor solution of Example 1. The precursor solution was applied tothe titanium sheets, previously dried on air at 50° C., by brushing in17 coats, with a subsequent decomposition cycle in forced airventilation oven at 510° C. for a time of 10 minutes after eachintermediate coat and of 60 minutes after the final coat.

A subsequent weight check evidenced the application of an externalcatalytic coating of 15 g/m² of noble metal, expressed as the sum of Irand Ru. The thus obtained electrodes were characterised in theaccelerated life-test of Example 1, allowing a constant cell operationat a cell voltage of about 3 V for approximately 230 hours, followed bya sudden voltage increase indicating the deactivation of the electrodesto an extent which forced the discontinuation of the test.

The previous description is not intended to limit the invention, whichmay be used according to different embodiments without departing fromthe scopes thereof, and whose extent is univocally defined by theappended claims.

Throughout the description and claims of the present application, theterm “comprise” and variations thereof such as “comprising” and“comprises” are not intended to exclude the presence of other elementsor additives.

The discussion of documents, acts, materials, devices, articles and thelike is included in this specification solely for the purpose ofproviding a context for the present invention. It is not suggested orrepresented that any or all of these matters formed part of the priorart base or were common general knowledge in the field relevant to thepresent invention before the priority date of each claim of thisapplication.

1. Electrode for electrolytic cell comprising a valve metal substrate,an internal catalytic coating and an external catalytic coating ofdifferent composition and higher activity overlaid thereto, theroughness profile of said substrate having an R_(a) value of 4 to 8 μmand an R_(z) value of 20 to 50 μm, said internal catalytic coatingcontaining oxides of iridium, ruthenium and a valve metal selectedbetween tantalum and niobium with an overall specific loading of iridiumand ruthenium expressed as metals of 2 to 5 g/m², said externalcatalytic coating containing noble metal oxides with a specific loadingnot lower than 7 g/m².
 2. The electrode according to claim 1, whereinsaid external catalytic coating comprises a mixture of oxides ofiridium, ruthenium and titanium with a molar concentration of rutheniumof 12-18%, a molar concentration of iridium of 6-10% and a molarconcentration of titanium of 72-82%.
 3. The electrode according to claim1, wherein said internal catalytic coating contains a mixture of oxidesof iridium, ruthenium and tantalum with a molar concentration ofruthenium of 42-52%, a molar concentration of iridium of 22-28% and amolar concentration of tantalum of 20-36%.
 4. The electrode according toclaim 1, further comprising a protective layer consisting of valve metaloxides interposed between said substrate and said internal catalyticcoating.
 5. Method for the production of an electrode comprising:thermally treating a valve metal substrate, the roughness profile ofsaid substrate having an R_(a) value of 4 to 8 μm and an R_(z) value of20 to 50 μm, for a time not lower than 4 hours at a temperature notlower than 560° C., acid etching, applying an internal catalytic coatingby thermal decomposition of a precursor solution, the internal catalyticcoating comprising oxides of iridium, ruthenium and a valve metalselected between tantalum and niobium with an overall specific loadingof iridium and ruthenium expressed as metals of 2 to 5 g/m²; applying anexternal catalytic coating by thermal decomposition of a precursorsolution, external catalytic coating containing noble metal oxides witha specific loading not lower than 7 g/m².
 6. The method according toclaim 5, wherein said acid etching step is carried out in 25-30% byweight sulphuric acid containing 5 to 10 g/l of dissolved titanium, at atemperature of 80 to 90° C. until reaching a weight loss not lower than180 g/m² of metal.
 7. The method according to claim 5 comprising, beforesaid internal catalytic coating application step, an additional step ofapplication of a protective layer consisting of valve metal oxides by atechnique selected between a thermal treatment of the substrate, a flameor plasma spray application and a thermal decomposition of a precursorsolution.
 8. Electrochemical cell for alkali metal hypochloriteproduction comprising an aqueous electrolyte containing alkali metalchlorides, at least one pair of electrodes, the electrodes eachcomprising a valve metal substrate, an internal catalytic coating and anexternal catalytic coating of different composition and higher activityoverlaid thereto, the roughness profile of said substrate having anR_(a) value of 4 to 8 μm and an R_(z) value of 20 to 50 μm, saidinternal catalytic coating containing oxides of iridium, ruthenium and avalve metal selected between tantalum and niobium with an overallspecific loading of iridium and ruthenium expressed as metals of 2 to 5g/m², said external catalytic coating containing noble metal oxides witha specific loading not lower than 7 g/m²; and a timed control system toalternately polarise said electrodes for a predetermined period of timedetermining the functioning thereof as anode and as cathoderespectively.
 9. The cell according to claim 8, wherein said aqueouselectrolyte has an ion chloride concentration of 2 to 20 g/l.
 10. Thecell according to claim 8, wherein said predetermined period of time is0.5 to 60 minutes.